Instead, it's something even scarier: The first instance of hackers serving up "scareware," fake security software that, once installed, nags users with so many alerts that some fork over $50 or more just to "register" the program and get rid of the warnings.

"This is just another scareware installer," Schouwenberg said, referring to the malware that's downloaded onto victimized PCs. "There's no worm component. But it's quite significant as it's the first time that Twitter's been used for a traditional type of attack."

Over the weekend, Twitter users began receiving tweets with the phrase "Best Video" and a link to a Russian domain. Although those who clicked on the link were directed to a site with a video, they were also served a malicious PDF document via an IFRAME on that site.

The PDF, said Schouwenberg, contains a number of exploits, and tries each in turn. If it's able to compromise the computer using one of those exploits, the malware then installs phony security software.

The PDF appears to contain attack code from "LuckySploit," a relatively-new multi-strike hacker toolkit that uses malicious JavaScript, said Schouwenberg.

Twitter's not able to remove any malware installed by the attacks, of course, leaving that chore up to users.

Schouwenberg's sure that Twitter's talk of cleaning accounts was a smokescreen, as unlike attacks in April, this one wasn't a worm. "There was no self-replicating code in the binary," he said. Instead, Schouwenberg believes that the malicious tweets were sent from Twitter accounts whose log-on credentials had been hijacked previously by basic phishing-style scams.

"When I first saw this Saturday night, I thought of the Twitter phishing attack, which was quite high profile," said Schouwenberg. "Phishing always has a greater purpose ... so when all of a sudden you see a new 'worm' but there's no worm component [in the attack code], it's clear that this was based on compromised accounts, rather than self-replicating."

Schouwenberg also found the links in the malicious tweets on multiple Web forums, giving credence to his theory that hijacked accounts were used to launch the scareware attack.

Twitter users should expect to see more such attacks, Schouwenberg said. "The whole idea of Twitter is to click on links," he said. "It's a security nightmare."